The recent development in the enhancement of integration density in the CSD type solid-state imaging device is eminent. The area occupied by an element is fine patterned, and with the progress of the fine patterning an enhancement in the sensitivity is required. As one satisfying such a requirement of high sensitivity, a charge sweeping type solid-state imaging device (hereinafter referred to as a "CSD imaging device") is disclosed in M. Kimata et al, "International Solid State Circuit Conference, Digest of Technical Papers" 1985 February, pp 100, in which the utility thereof is also confirmed.
FIG. 3 shows a block diagram of a CSD type imaging device, and FIG. 4(a) shows the operation concept thereof. That is, the cross section of the CSD element in the vertical direction and the potential distribution thereof are typically illustrated.
In FIG. 3, the reference numeral 103 designates a light-electricity conversion element such as a photodiode, the numeral 101 designates a transfer gate for reading out the charges from the photodiode 103, the numeral 105 designates a CSD channel, and the numeral 102 designates a CSD transfer electrode. The transfer gate 101 and the CSD transfer electrode 102 are usually constituted by the same electrode. The numeral 104 designates a vertical scanning line and all the CSD transfer electrodes 102 are combined by the vertical scanning line 104 in the horizontal direction. The numeral 106 designates a storage gate (ST), the numeral 107 designates a storage control gate, the numeral 108 designates an output section, and the numeral 109 designates a horizontal CCD.
The operation of this CSD type imaging device will be described with reference to FIG. 4.
At first, when a transfer gate is selected by the TG scanning circuit 2 during the horizontal blanking period, signal charges (Qsig) are read out to the CSD channel 105. In this figure, the vertical scanning line 104c is selected and the charges Qsig are read out from the hatched photodiode 103 (refer to 1 of FIG. 4(a)). Next, the signal charges Qsig are transferred by CSD in the horizontal reading out period (refer to 2 to 4 of FIG. 4(a)). Then, the CSD operates similarly as a four phase CCD, driven by the CSD scanning circuit 1. The signal charges Qsig are collected at the storage gate 106 by such an operation, and transferred to the horizontal CCD 109 through the storage control gate 107 in the next horizontal blanking period (refer to 6 of FIG. 4(a)). FIG. 4(b) typically shows the timing chart of the picture element selection signals applied to the vertical scanning line 104a to 104h by the TG scanning circuit 2. The respective vertical scanning line 104 is selected only one time in one field.
By the way, an electronic shutter function of 1/1000 second, that is, a function of viewing the imaged picture with stopping the same at about 1/1000 second is required in image sensors recently. In the prior art solid-state imaging device, however, it takes 1/60 second to read out one frame of a picture. The light which is input during that time period is integrated as the output signal. An example which has realized the above-described electronic shutter function is an interline type CCD (ILCCD) sensor. In this ILCCD the light detector is for a while and the light signal of only about 1 ms duration is stored at the light detector during the vertical blanking period of about 1 ms. Thereafter this stored signal is read out to the vertical CCD, thereby realizing an exposure time of 1/1000 second.
In this way, in an ILCCD a shutter operation can be easily realized because the vertical CCD operation is equivalent to those of memory elements corresponding to one picture frame. Furthermore, the excess charges stored in a time period other than the shutter period can be swept out during the vertical blanking period for storing the signal charges by the use of the vertical CCD independently of the operation of the photodiode.
On the other hand, in the above-described CSD type imaging device, picture element selections are conducted successively during the vertical scanning period, and this leads to a difficulty in charges while keeping the exposure time constant.